(450a) Keynote: The Remarkable Influence of Zr-Doping on the Thermochemical Properties of Ceria

Ceria is a well-known material for the production of hydrogen by thermochemical cycling. While ceria offers several advantageous properties such as rapid surface reaction rate, high bulk oxygen diffusivity, and excellent stability over hundreds of redox cycles, it suffers from the requirement of reduction at relatively high temperatures (preferably as high as 1500 °C) in order to generate non-negligible quantities of hydrogen fuel per cycle. To address this challenge, several laboratories have evaluated zirconia-substituted ceria as an alternative to neat ceria. Introduction of zirconia has the surprising consequence of increasing the oxygen non-stoichiometry of the host oxide, despite the fixed 4+ valence of Zr. This behavior is reflected in a decrease in both the enthalpy and the entropy of reduction of (Ce_1-xZr_x)O_2-δ with increasing x, as determined by thermogravimetric analysis of bulk, porous samples. The thermodynamic properties suggest enhanced efficiency for solar-to-fuel conversion as well as the possibility of cycling at overall reduced temperatures without loss in hydrogen production rates. The kinetic properties, however, appear to be negatively impacted by introduction of Zr. Using angle-resolved X-ray Absorption Near Edge Spectroscopy (XANES), we quantify under technologically relevant conditions the Ce³⁺ concentration in the surface (2-3 nm), as well as bulk regions, of ceria-zirconia films. We find that the surface is in all cases more reduced than the bulk, however, with increasing Zr, the differential between the two regions diminishes. In parallel, using electrical conductivity relaxation methods, we find that both the surface reaction rate and the bulk chemical diffusion of oxygen decreases in the presence of Zr. These observations are tentatively explained in terms of the preferred lower coordination of the small Zr⁴⁺ ion relative to the larger Ce⁴⁺ and Ce³⁺ ions and likely trapping of oxygen vacancies in the vicinity of Zr. Despite these effects, zirconia substitution remains attractive because of the overwhelming importance of thermodynamic properties.